Digital representation, storage, distribution, and duplication of digital media have become very popular because they are inexpensive, easy to use and maintain the quality of the media. These advantages however have enabled widespread, illegal distribution and use of copyrighted material, such as unauthorized distribution of digital images and videos over the Internet.
Many different approaches have been presented to secure digital media against unauthorized use. For example, digital encryption technology is effective to enable secure communication and delivery. However, if the encrypted bitstream is decrypted or presented in a form that is visible or audible to humans, that content can be re-recorded and an unsecured copy can be obtained.
Marking media by embedding recipient information in the media can help identify individuals that receive the media and use the content in an unauthorized manner. Further, embedded ownership information in the media can indicate copy restriction and clarify ownership of the media.
One way of marking media is by adding annotations to the digital media file format that can be read from the unmodified file and that are ignored during playback. This information is most often lost, however, if the file is re-recorded or converted to another format.
To achieve a robust and permanent mark in video media, visible and overlay images that display copyright information during playback have been proposed. Overlaid images are robust against modification and easy to read. However, this approach can reduce the quality of the marked video, because the overlaid images interfere with the original video, which diminishes the quality of the viewing experience. In addition, overlaid images are obvious and are therefore easy to identify, and can be removed by overwriting or cropping.
Digital watermarking is typically understood as the embedding of imperceptible, robust and secure information in media content. In the prior art, many different methods have been proposed on how to accomplish the embedding of a digital mark. While some methods provide a fairly strong solution in terms of robustness and imperceptibility, the challenge of efficient application remains. Watermarking typically requires modification of many elements that encode data, such as audio samples or pixels in a frame or image of the media object, which is processing intensive. In addition, many watermarking applications require the modification of many media objects, for example to enable distribution of unique copies of the same content to millions of recipients. To uniquely process many media objects before delivery, a very efficient process for applying a watermark is desirable. Alternatively, a marking process may be applied by a receiving device. These devices however are often very limited in available processing resources and again, a very efficient system for performing watermarking is desirable.
Direct modification of the bitstream representing the compressed (also called encoded) content is required in applications that require embedding without re-compression (re-encoding). These applications do not allow access to the decoded video content because a decode, marking and re-encode of the content is too processing intensive, introduces processing delay and often reduces the quality of the video at least slightly. Examples of applications where embedding information in compressed content is desirable include embedding on a server that distributes uniquely marked copies to recipients in compressed form and applications on the receiving end where the compressed content is marked before storage, or fed into a hardware decoder and display engine that can not readily be modified to add watermarking functionality after the content is decoded.
Manipulation of compressed content provides a particular challenge, because it is complex to predict how a modification will affect the resulting decoded content, since multiple operations are applied that convert the compressed content to the uncompressed content. Another challenge is the underlying compression principle of re-using prior encoded information and thereby reducing redundancy. This means that elements that are modified are often re-used and modification will therefore propagate to additional locations, often resulting in unwanted effects that interfere with the desired imperceptibility of the embedded information. In prior art, concepts have been described that work on partly compressed content in that some stages of the compression, such as the entropy coding, are performed before modifications can be applied. While this offers some advantages in processing, it is often insufficient to comply with very limited processing restrictions present in many applications.